Determining which implementation to inject at runtime using .NET Core dependency injection

Without Using a Separate IoC Container

Here's an approach that's way easier than configuring your app to use another IoC container and then configuring that container. After working through this with Windsor this solution seems a whole lot easier.

This approach is simplest if you can use a singleton instance of each service implementation.

We'll start with an interface, some implementations, and the factory we can inject which will return an implementation selected at runtime based on some input.

public interface ICustomService { }
public class CustomServiceOne : ICustomService { }
public class CustomServiceTwo : ICustomService { }
public class CustomServiceThree : ICustomService { }

public interface ICustomServiceFactory
{
    ICustomService Create(string input);
}

Here's a really crude implementation of the factory. (Didn't use string constants, or polish it at all.)

public class CustomServiceFactory : ICustomServiceFactory
{
    private readonly Dictionary<string, ICustomService> _services 
        = new Dictionary<string, ICustomService>(StringComparer.OrdinalIgnoreCase);

    public CustomServiceFactory(IServiceProvider serviceProvider)
    {
        _services.Add("TypeOne", serviceProvider.GetService<CustomServiceOne>());
        _services.Add("TypeTwo", serviceProvider.GetService<CustomServiceTwo>());
        _services.Add("TypeThree", serviceProvider.GetService<CustomServiceThree>());
    }

    public ICustomService Create(string input)
    {
        return _services.ContainsKey(input) ? _services[input] : _services["TypeOne"];
    }
}

This assumes that you've already registered CustomServiceOne, CustomServiceTwo, etc. with the IServiceCollection. They would not be registered as interface implementations, since that's not how we're resolving them. This class will simply resolve each one and put them in a dictionary so that you can retrieve them by name.

In this case the factory method takes a string, but you could inspect any type or multiple arguments to determine which implementation to return. Even the use of a string as the dictionary key is arbitrary. And, just as an example, I provided fallback behavior to return some default implementation. It might make more sense to throw an exception instead if you can't determine the right implementation to return.

Another alternative, depending on your needs, would be to resolve the implementation within the factory when it's requested. To the extent possible I try to keep most classes stateless so that I can resolve and reuse a single instance.

To register the factory with the IServiceCollection at startup we would do this:

services.AddSingleton<ICustomServiceFactory>(provider => 
    new CustomServiceFactory(provider));

The IServiceProvider will be injected into the factory when the factory is resolved, and then the factory will use it to resolve the service.

Here's the corresponding unit tests. The test method is the identical to the one used in the Windsor answer, which "proves" that we can transparently replace one factory implementation with another and change other stuff in the composition root without breaking stuff.

public class Tests
{
    private IServiceProvider _serviceProvider;
    [SetUp]
    public void Setup()
    {
        var services = new ServiceCollection();
        services.AddSingleton<CustomServiceOne>();
        services.AddSingleton<CustomServiceTwo>();
        services.AddSingleton<CustomServiceThree>();
        services.AddSingleton<ICustomServiceFactory>(provider => 
            new CustomServiceFactory(provider));
        _serviceProvider = services.BuildServiceProvider();
    }

    [TestCase("TypeOne", typeof(CustomServiceOne))]
    [TestCase("TypeTwo", typeof(CustomServiceTwo))]
    [TestCase("TYPEThree", typeof(CustomServiceThree))]
    [TestCase("unknown", typeof(CustomServiceOne))]
    public void FactoryReturnsExpectedService(string input, Type expectedType)
    {
        var factory = _serviceProvider.GetService<ICustomServiceFactory>();
        var service = factory.Create(input);
        Assert.IsInstanceOf(expectedType, service);
    }
}

As in the Windsor example, this is written to avoid any reference to the container outside of the composition root. If a class depends on ICustomServiceFactory and ICustomService you could switch between this implementation, the Windsor implementation, or any other implementation of the factory.

Using Windsor

I'm going to sidestep the questions about whether or not this makes sense in this case and just attempt to answer the question as asked:

.NET Core's IoC container isn't built particularly well for this sort of scenario. (They acknowledge this in their documentation.) You can work around it by adding another IoC container like Windsor.

The implementation ended up looking way more complicated than I would have liked, but once you get past the setup it's not bad and you get access to Windsor's features. I'm going to provide another answer that doesn't include Windsor. I had to do all of this work to see that I probably like the other approach better.

In your project, add the Castle.Windsor.MsDependencyInjection NuGet package.

Interfaces and Implementations for Testing

For testing, I added some interfaces and implementations:

public interface ICustomService { }
public interface IRegisteredWithServiceCollection { }
public class CustomServiceOne : ICustomService { }
public class CustomServiceTwo : ICustomService { }
public class CustomServiceThree : ICustomService { }
public class RegisteredWithServiceCollection : IRegisteredWithServiceCollection { }

The intent is to create a factory that will select and return an implementation of ICustomService using some runtime input.

Here's an interface which will serve as a factory. This is what we can inject into a class and call at runtime to get an implementation of ICustomService:

public interface ICustomServiceFactory
{
    ICustomService Create(string input);
}

Configure the Windsor Container

Next is a class which will configure an IWindsorContainer to resolve dependencies:

public class WindsorConfiguration : IWindsorInstaller
{
    public void Install(IWindsorContainer container, IConfigurationStore store)
    {
        container.AddFacility<TypedFactoryFacility>();
        container.Register(
            Component.For<ICustomService, CustomServiceOne>().Named("TypeOne"),
            Component.For<ICustomService, CustomServiceTwo>().Named("TypeTwo"),
            Component.For<ICustomService, CustomServiceThree>().Named("TypeThree"),
            Component.For<ICustomService, CustomServiceOne>().IsDefault(),
            Component.For<ICustomServiceFactory>().AsFactory(new CustomServiceSelector())
        );
    }
}

public class CustomServiceSelector : DefaultTypedFactoryComponentSelector
{
    public CustomServiceSelector()
        : base(fallbackToResolveByTypeIfNameNotFound: true) { }

    protected override string GetComponentName(MethodInfo method, object[] arguments)
    {
       return (string) arguments[0];
    }
}

Here's what's going on in here:

• The TypedFactoryFacility will enable us to use Windsor's typed factories. It will create an implementation of our factory interface for us.
• We're registering three implementations of ICustomService. Because we're registering more than one implementation, each must have a name. When we resolve ICustomService we can specify a name, and it will resolve the type according to that string.
• For illustration I registered another implementation of ICustomService without a name. That will enable us to resolve a default implementation if we try to resolve using an unrecognized name. (Some alternatives are just throwing an exception, or returning a "null" instance of ICustomService or creating a class like UnknownCustomService that throws an exception.)
• Component.For<ICustomServiceFactory>().AsFactory(new CustomServiceSelector()) tells the container to create a proxy class to implement ICustomServiceFactory. (More on that in their documentation.)
• CustomServiceSelector is what takes the argument passed to the factory's Create method and returns the component name (TypeOne, TypeTwo, etc.) that will be used to select a component. In this case we're expecting that the argument passed to the factory will be the same as the registration name we've used. But we could replace this with other logic. Our factory could even take arguments of other types which we could inspect and determine which string to return.

Configure Your App To Use the Windsor Container

Now, in StartUp, modify ConfigureServices to return IServiceProvider instead of void and create an IServiceProvider that combines services registered directly with the IServiceCollection with those registered with the Windsor container:

public IServiceProvider ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
    var container = new WindsorContainer();
    container.Install(new WindsorConfiguration());
    return WindsorRegistrationHelper.CreateServiceProvider(container, services);
}

container.Install(new WindsorConfiguration()) allows WindsorConfiguration to configure our container. We could just configure the container right in this method, but this is a nice way to keep our container configurations organized. We can create numerous IWindsorInstaller implementations or our own custom classes to configure the Windsor container.
WindsorRegistrationHelper.CreateServiceProvider(container, services) creates the IServiceProvider that uses container and services.

Does It Work?

I wouldn't post all this without finding out first. Here's some NUnit tests. (I usually write some basic tests for DI configuration.)

The setup creates an IServiceProvider similar to what would happen in the application startup. It creates a container and applies the WindsorConfiguration. I'm also registering a service directly with the ServiceCollection to make sure that the two play well together. Then I'm combining the two into an IServiceProvider.

Then I'm resolving an ICustomerServiceFactory from the IServiceProvider and verifying that it returns the correct implementation of ICustomService for each input string, including the fallback when the string isn't a recognized dependency name. I'm also verifying that the service registered directly with ServiceCollection is resolved.

public class Tests
{
    private IServiceProvider _serviceProvider;
    [SetUp]
    public void Setup()
    {
        var services = new ServiceCollection();
        services.AddSingleton<IRegisteredWithServiceCollection, RegisteredWithServiceCollection>();
        var container = new WindsorContainer();
        container.Install(new WindsorConfiguration());
        _serviceProvider = WindsorRegistrationHelper.CreateServiceProvider(container, services);
    }

    [TestCase("TypeOne", typeof(CustomServiceOne))]
    [TestCase("TypeTwo", typeof(CustomServiceTwo))]
    [TestCase("TYPEThree", typeof(CustomServiceThree))]
    [TestCase("unknown", typeof(CustomServiceOne))]
    public void FactoryReturnsExpectedService(string input, Type expectedType)
    {
        var factory = _serviceProvider.GetService<ICustomServiceFactory>();
        var service = factory.Create(input);
        Assert.IsInstanceOf(expectedType, service);
    }

    [Test]
    public void ServiceProviderReturnsServiceRegisteredWithServiceCollection()
    {
        var service = _serviceProvider.GetService<IRegisteredWithServiceCollection>();
        Assert.IsInstanceOf<RegisteredWithServiceCollection>(service);
    }
}

Is All of This Worth It?

Now that I've figured it out, I'd probably use it if I really needed this sort of functionality. It looks worse if you're trying to assimilate both using Windsor with .NET Core and seeing its abstract factory implementation for the first time. Here's another article with some more information on Windsor's abstract factory without all the noise about .NET Core.

I am going to go out on a limb here and say that the attempt to utilize dependency injection for this purpose is sub-optimal. Normally this would be handled by a Factory pattern that produces service implementations using the dreaded if and switch statements. A simple example is:

public interface IPhotoService { 
     Photo CreatePhoto(params);
}

public class PhotoServiceFactory {
    private readonly IPhotoService _type1;
    private readonly IPhotoService _type2;
    private readonly IPhotoService _type3;
    public PhotoServiceFactory(IDependency1 d1, IDependency2 d2, ...etc) {
        _type1 = new ConcreteServiceA(d1);
        _type2 = new ConcreteServiceB(d2);
        _type3 = new ConcreteServiceC(etc);
    }
    public IPhotoService Create(User user) {
        switch(user.Claim) {
            case ClaimEnum.Type1:
                return _type1;
            case ClaimEnum.Type2:
                return _type2;
            case ClaimEnum.Type3:
                return _type3;
            default:
                throw new NotImplementedException
        }
    }
}

Then in your controller:

public class PhotosController {
    IPhotoServiceFactory _factory;
    public PhotosController(IPhotoServiceFactory factory){
       _factory = factory;
    } 
    public IHttpActionResult GetPhoto() {
       var photoServiceToUse = _factory.Create(User);
       var photo = photoServiceToUse.CreatePhoto(params);
       return Ok(photo);
    }
}

Alternately just use the concrete classes as arguments in the constructor and follow a similar logic as to the above.

Here is one solution, i have created inside asp.net core console application.

using System;
using System.Collections.Generic;
using Microsoft.Extensions.DependencyInjection;

namespace CreationalPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            // Add dependency into service collection
            var services = new ServiceCollection()
                .AddTransient<FordFigoFactory>()
                .AddTransient<AudiQ7Factory>();

            /* Create CarServiceFactory as singleton because it can be used across the application more frequently*/
            services.AddSingleton<ICarServiceFactory>(provider => new CarServiceFactory(provider));            

            // create a service provider from the service collection
            var serviceProvider = services.BuildServiceProvider();


            /* instantiate car*/
            var factory = serviceProvider.GetService<ICarServiceFactory>();
            var audiCar = factory.Create("audi").CreateACar("Blue");            

            Console.Read();
        }
    }


    public interface ICarServiceFactory
    {
        ICreateCars Create(string input);
    }

    public class CarServiceFactory : ICarServiceFactory
    {
        private readonly Dictionary<string, ICreateCars> _services
            = new Dictionary<string, ICreateCars>(StringComparer.OrdinalIgnoreCase);

        public CarServiceFactory(IServiceProvider serviceProvider)
        {
            _services.Add("ford", serviceProvider.GetService<FordFigoFactory>());
            _services.Add("audi", serviceProvider.GetService<AudiQ7Factory>());               
        }

        public ICreateCars Create(string input)
        {
            Console.WriteLine(input + " car is created.");
            return _services.ContainsKey(input) ? _services[input] : _services["ford"];
        }
    }


    public interface ICreateCars
    {
        Car CreateACar(string color);        

    }

    public class FordFigoFactory : ICreateCars
    {
        public Car CreateACar(string color)
        {
            Console.WriteLine("FordFigo car is created with color:" + color);

            return new Fordigo { Color = color};
        }
    }

    public class AudiQ7Factory : ICreateCars
    {
        public Car CreateACar(string color)
        {
            Console.WriteLine("AudiQ7 car is created with color:" + color);

            return new AudiQ7 { Color = color };
        }
    }

    public abstract class Car
    {
        public string Model { get; set; }
        public string Color { get; set; }
        public string Company { get; set; }        
    }

    public class Fordigo : Car
    {
        public Fordigo()
        {
            Model = "Figo";
            Company = "Ford";
        }
    }

    public class AudiQ7 : Car
    {
        public AudiQ7()
        {
            Model = "Audi";
            Company = "Q7";
        }
    }
}

Explanation: To understand better try to read the program from bottom to top. We have 3 sections:

1. Car (Car, Fordigo, AudiQ7)
2. CarFactory (ICreateCars, FordFigoFactory, AudiQ7Factory)
3. CarService (ICarServiceFactory, CarServiceFactory)

In this Dependency injection is registered as transient for Factory classes FordFigoFactory and AudiQ7Factory. And Singleton for CarServiceFactory.